What lies beyond a black hole’s event horizon is a mystery.
It’s the boundary that marks the distance at which light can no longer escape a black hole – leaving us striving for answers about what a black hole actually looks like without a way to see inside.
For simplicity’s sake, textbooks usually describe the singularity hidden beyond the event horizon as a zero-dimensional point of infinite density.
According to a new paper set to be published in Physical Review D, that simplification may have taken a step too far.
Instead, theoretical physicists Andrew J. S. Hamilton of the University of Colorado Boulder and Tyler McMaken of the University of Mary argue that the singularity is better understood as a flat, three-dimensional surface.
“I joke to my students that black holes always defy my intuition,” Hamilton told ScienceAlert.
“Yes, it was a surprise. At the same time, it made sense of the flat appearance of the Schwarzschild singularity.”
Inside a black hole, space falls faster than light towards the singularity. By the 1930s, physicists had come to picture that singularity as a point.
Then, in the 1960s, the picture started to change. In a Kerr black hole – one that rotates – physicists found that the motion of that fall towards the singularity is slowed by centrifugal repulsion, with the end result that the singularity is not a point, but a ring.
For a Schwarzschild black hole – one that is not rotating – the singularity remained a point. It’s easy to draw, easy to teach, and relatively easy to understand, so the model persisted.
However, even decades ago, Hamilton had begun to develop an uncomfortable inkling that the point was a poor representation of Schwarzschild black hole physics.
“Quantum gravity is THE central question of theoretical physics, and the singular surfaces of black holes are where quantum gravity happens in our Universe.” – theoretical physicist Andrew J. S. Hamilton
It was 1998, and the curiosity of Hamilton’s students in an introductory astronomy class led him to build general-relativistic visualizations of what it might be like to fall into a black hole.
What came out was… a surface.
“I reported what I saw, but did not try to explain it, because I did not understand it,” he said.
“The evidence shows that yes, we hesitated. The journey of understanding has taken decades.”
The answer, Hamilton realized, lay in following two observers as they fell into the same black hole from opposite directions.
The question was simple: Where do they end up?
If the singularity were a point, both observers should eventually end up at the same location.
But according to general relativity, Hamilton and McMaken show, that can never happen.
As the observers fall, the black hole’s warped spacetime prevents light – or any other signal – from traveling between them. They become permanently causally disconnected, meaning neither can ever influence or even know about the other’s final moments.
Hamilton and McMaken argue that if the observers can no longer exchange information before reaching the singularity, they cannot ultimately converge on the same spacetime event.
The contradiction disappears if the singularity is a three-dimensional spacelike surface that each observer reaches at a different location.
Or, as Hamilton put it in 2010: “‘The’ singularity is not a point. Rather, it is a three-dimensional spatial boundary where general relativity commits suicide.”
The work doesn’t just apply to Schwarzschild black holes. The ring singularities of Kerr black holes have long been considered fundamentally different from Schwarzschild points, but maybe the two aren’t so different after all.
Instead, the authors argue that a realistic Kerr black hole would collapse into a spacelike singular surface at its inner horizon, making its singularity more like that of a Schwarzschild black hole than the familiar textbook picture suggests.
That means the work could reshape how physicists think about singularities from basic principles.
If the Schwarzschild singularity is a surface rather than a point, that changes the mathematical setting in which general relativity breaks down – and therefore the starting point for any future theory of quantum gravity.
“Quantum gravity is THE central question of theoretical physics, and the singular surfaces of black holes are where quantum gravity happens in our Universe,” Hamilton explained.
“Much of the literature on quantum gravity is abstract and disconnected from reality. We would love to see the community evolve in more realistic directions.”
Related: Radical Theory Says Black Holes May Spew Matter And Time as White Holes
Quantum gravity is the long-sought theory that would unite Einstein’s general relativity, which describes gravity on cosmic scales, with quantum mechanics, which governs the quantum world on atomic and subatomic scales.
Both theories work extraordinarily well on their own, but they become mathematically incompatible under the extreme conditions inside a black hole.
“A lofty goal of modern theoretical physics is a viable theory of quantum gravity. It’s not there yet,” Hamilton said.
“Our hope is that we can make progress by exploring the places where quantum gravity is just about to happen.”
The research has been accepted for publication in Physical Review D.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
